Phosphorus Availability from Swine and Dairy Slurries

Compared to nitrogen (N), relatively little is known about the availability of phosphorus (P) from manure to crops. Many state extension bulletins recommend that 50-80% of total P in manure is available to crops the first year after application. This number is likely based on the fact that 35-90% of P in manures is in the inorganic form (Peperzak, et al., 1959; Barnett, 1994), which is immediately available to crops. The remaining P that is not credited in the first year is usually never credited. In addition, the same availability coefficient/index is usually given to all manure types, regardless of species, diets, or storage. Understanding manure P chemistry in soil and trying to predict P availability is an important component for making nutrient management plans that maximize economic benefits and minimize environmental risks.